1. Field of the Invention
The invention relates to a method for supplying voltage to a sensor from energy sources having little energy, and to devices for use of the method. The following statements are based on a Geiger-Muller counting tube for detection of radioactive radiation, as an example of such a sensor. However, it should be mentioned that the invention is not limited to sensors, but may also be used, for example, for supplying voltage to other loads, particularly if such loads draw only a very small amount of current at a voltage of more than 100 volts.
2. Description of the Related Art
A number of methods are known for supplying voltage to sensors. These can be divided into open-loop and closed-loop controlled voltage converters.
Typical open-loop controlled voltage supplies include single-ended forward converters and push-pull choppers, in which the primary voltage is converted to the secondary voltage with a fixed transformation ratio.
Single-ended flyback converters store a certain amount of energy in the inductor during a primary pulse, which switches on a switching transistor or some other suitable switch for the duration of the primary pulse, and thus produce a rising current in a circuit which essentially comprises a primary energy source, for example a battery, an inductor and a switch, which energy is emitted on the secondary side via a rectifier once the primary pulse has been switched off. If the load on the secondary side is known, it is thus also possible to control the secondary voltage that is achieved, for example by taking account of the power that results from the load on the secondary side by suitably defining the primary pulse rate and primary pulse duration. If there is no load on the secondary side, the voltage theoretically rises in an unlimited fashion, but in practice it is limited by a charge flowing back owing to the rectifier not having a blocking response, and/or owing to the limited withstand voltage of the circuit, caused, for example, by the switching transistor having an inadequate blocking capability, or by voltage flashovers in the inductor or in any energy-storage capacitor that is used. If the load on the secondary side is unknown, then, in order to avoid circuit malfunctions or damage to components, the secondary voltage is generally regulated or limited, particularly if it is intended to be at a specific value or, at least, is not intended to exceed such a value. The secondary voltage is also often limited by parasitic characteristics of components, such as the avalanche effect of diodes, the capacitance of the inductor, and the reverse current of the rectifier. While a voltage supply according to the prior art offers satisfactory efficiency for a load drawing a medium or high amount of current, this is not the case for a load drawing a particularly low current.
Geiger-Muller counting tubes are loads which, for optimum measurement accuracy, must be supplied with a constant voltage over a wide required current range. For example, the current required for a typical counting tube for an optimum measurement voltage of 500 volts when there is no radiation is 0.25 nA, but is 15 .mu.A for maximum radiation, corresponding to a ratio of 1:60,000.
For example, G. Peltz, Single-coil voltage converters from 9 V to 150 V, Electronics 24/1981, page 76, describes how a supply current of more than 2 milliamperes is drawn from the primary energy source even for a low voltage conversion ratio with a factor of about 15, even when there is no load. This article describes how high-frequency burst rates are used to produce the output voltage, and what is meant by the term "good efficiency" according to the prior art. For example, the efficiency is stated to be 70 percent for an output load of one milliampere.
The previously known methods are subject to many problems which severely load the primary energy source, for example a battery, and thus limit the possible operating period, since the current supplies to test equipment for measuring radioactive radiation using a Geiger-Muller counting tube as the sensor cannot provide for the small amount of current required by the sensor when the radiation is low, with high efficiency. This is particularly true if it is also intended that the supply voltage for the sensor should not collapse at maximum radiation.
Thus, for battery-operated test equipment, it is impossible to measure and record the radiation over a lengthy period, without having to replace or charge the battery regularly.
With regard to the prior art, reference should also be made to the following literature. It is known from JP 59117459 for a plurality of diodes to be connected in series in order to be able to use such a low reverse voltage, and EP-A-0 189 340 likewise discloses a parallel circuit of two diodes, in order to reduce oscillations during the change to the reverse-biased zone. On the other hand, neither circuit allows the reverse current to be reduced.